If you want to extract screenshots that you have taken or install mods for your Minecraft, you have to find the .minecraft folder. In linux, there are three ways of opening it:
Open a file browser and navigate to your home folder. This is normally shown as a folder called your username. Once you are there (assumuing Ubuntu) you need to press Ctrl-H. If you look through all the folders in the file browser now you should see one called .minecraft. If you go inside, you have found the folder and can do whatever you wanted to do.
If you don't want to use a graphical file browser or don't run Ubuntu, you can use a terminal emulator to get there. Simply run the command:
And you will have successfully navigated to your .minecraft folder using the terminal.
If you already have Minecraft open, you can go almost directly to the .minecraft folder. You can go into the options menu, click on the texture packs button, then click the open texture pack directory button. After you've done that, a file manager window should open (this may take a while). Inside, go up a directory and you will be in your .minecraft folder!